Showing papers on "Terrane published in 2017"

Quantifying the rise of the Himalaya orogen and implications for the South Asian monsoon

Abstract: We reconstruct the rise of a segment of the southern flank of the Himalaya-Tibet orogen, to the south of the Lhasa terrane, using a paleoaltimeter based on paleoenthalpy encoded in fossil leaves from two new assemblages in southern Tibet (Liuqu and Qiabulin) and four previously known floras from the Himalaya foreland basin. U-Pb dating of zircons constrains the Liuqu flora to the latest Paleocene (ca. 56 Ma) and the Qiabulin flora to the earliest Miocene (21- 19 Ma). The proto-Himalaya grew slowly against a high (similar to 4 km) proto-Tibetan Plateau from similar to 1 km in the late Paleocene to similar to 2.3 km at the beginning of the Miocene, and achieved at least similar to 5.5 km by ca. 15 Ma. Contrasting precipitation patterns between the Himalaya-Tibet edifice and the Himalaya foreland basin for the past similar to 56 m.y. show progressive drying across southern Tibet, seemingly linked to the uplift of the Himalaya orogen.

Earth's first stable continents did not form by subduction

Abstract: Phase equilibria modelling of rocks from Western Australia confirms that the ancient continental crust could have formed by multistage melting of basaltic ‘parents’ along high geothermal gradients—a process incompatible with modern-style subduction. Tim Johnson et al. perform phase equilibria modelling of the Coucal basalts from Western Australia and confirm their suitability as parent rocks of the Archaean continental crust. The authors suggest that these early crustal rocks were produced by 20–30 per cent melting along high geothermal gradients. They conclude that the production and stabilization of the first continents required a protracted, multistage process. When coupled with the high geothermal gradients, this suggests that the continents did not form by subduction. Instead it favours a 'stagnant lid' regime in the early Archaean eon in which a single, rigid plate lay over the mantle. The geodynamic environment in which Earth’s first continents formed and were stabilized remains controversial1. Most exposed continental crust that can be dated back to the Archaean eon (4 billion to 2.5 billion years ago) comprises tonalite–trondhjemite–granodiorite rocks (TTGs) that were formed through partial melting of hydrated low-magnesium basaltic rocks2; notably, these TTGs have ‘arc-like’ signatures of trace elements and thus resemble the continental crust produced in modern subduction settings3. In the East Pilbara Terrane, Western Australia, low-magnesium basalts of the Coucal Formation at the base of the Pilbara Supergroup have trace-element compositions that are consistent with these being source rocks for TTGs. These basalts may be the remnants of a thick (more than 35 kilometres thick), ancient (more than 3.5 billion years old) basaltic crust4,5 that is predicted to have existed if Archaean mantle temperatures were much hotter than today’s6,7,8. Here, using phase equilibria modelling of the Coucal basalts, we confirm their suitability as TTG ‘parents’, and suggest that TTGs were produced by around 20 per cent to 30 per cent melting of the Coucal basalts along high geothermal gradients (of more than 700 degrees Celsius per gigapascal). We also analyse the trace-element composition of the Coucal basalts, and propose that these rocks were themselves derived from an earlier generation of high-magnesium basaltic rocks, suggesting that the arc-like signature in Archaean TTGs was inherited from an ancestral source lineage. This protracted, multistage process for the production and stabilization of the first continents—coupled with the high geothermal gradients—is incompatible with modern-style plate tectonics, and favours instead the formation of TTGs near the base of thick, plateau-like basaltic crust9. Thus subduction was not required to produce TTGs in the early Archaean eon.

Earth's first stable continents did not form by subduction

Abstract: Phase equilibria modelling of rocks from Western Australia confirms that the ancient continental crust could have formed by multistage melting of basaltic ‘parents’ along high geothermal gradients—a process incompatible with modern-style subduction Tim Johnson et al perform phase equilibria modelling of the Coucal basalts from Western Australia and confirm their suitability as parent rocks of the Archaean continental crust The authors suggest that these early crustal rocks were produced by 20–30 per cent melting along high geothermal gradients They conclude that the production and stabilization of the first continents required a protracted, multistage process When coupled with the high geothermal gradients, this suggests that the continents did not form by subduction Instead it favours a 'stagnant lid' regime in the early Archaean eon in which a single, rigid plate lay over the mantle The geodynamic environment in which Earth’s first continents formed and were stabilized remains controversial1 Most exposed continental crust that can be dated back to the Archaean eon (4 billion to 25 billion years ago) comprises tonalite–trondhjemite–granodiorite rocks (TTGs) that were formed through partial melting of hydrated low-magnesium basaltic rocks2; notably, these TTGs have ‘arc-like’ signatures of trace elements and thus resemble the continental crust produced in modern subduction settings3 In the East Pilbara Terrane, Western Australia, low-magnesium basalts of the Coucal Formation at the base of the Pilbara Supergroup have trace-element compositions that are consistent with these being source rocks for TTGs These basalts may be the remnants of a thick (more than 35 kilometres thick), ancient (more than 35 billion years old) basaltic crust4,5 that is predicted to have existed if Archaean mantle temperatures were much hotter than today’s6,7,8 Here, using phase equilibria modelling of the Coucal basalts, we confirm their suitability as TTG ‘parents’, and suggest that TTGs were produced by around 20 per cent to 30 per cent melting of the Coucal basalts along high geothermal gradients (of more than 700 degrees Celsius per gigapascal) We also analyse the trace-element composition of the Coucal basalts, and propose that these rocks were themselves derived from an earlier generation of high-magnesium basaltic rocks, suggesting that the arc-like signature in Archaean TTGs was inherited from an ancestral source lineage This protracted, multistage process for the production and stabilization of the first continents—coupled with the high geothermal gradients—is incompatible with modern-style plate tectonics, and favours instead the formation of TTGs near the base of thick, plateau-like basaltic crust9 Thus subduction was not required to produce TTGs in the early Archaean eon

Late Paleozoic closure of the Ob-Zaisan Ocean along the Irtysh shear zone (NW China): Implications for arc amalgamation and oroclinal bending in the Central Asian orogenic belt

Abstract: The largest accretionary orogen in the world, the Central Asian orogenic belt, has evolved through the assembly of various oceanic and continental blocks. Understanding the processes associated with the development of this orogenic belt relies on precise recognition of the boundaries between various terranes. One such major suture zone, which records the collisional history of the Siberian marginal arc system (Chinese Altai) with intra-oceanic arc systems (East/West Junggar), is the Irtysh shear zone in NW China. The spatial continuity and the tectonic nature of this shear zone are still poorly understood, but its development has supposedly made a significant impact on the architecture of the western Central Asian orogenic belt and the formation of the Kazakhstan orocline. Here, we provide new insight into the evolution of this shear zone based on detrital zircon ages, Hf isotope composition, structural data and Ar/Ar age constraints on the timing of deformation. Our results show a major discrepancy in detrital zircon populations and Hf model ages across the southern Chinese Altai and the northern East/West Junggar, thus allowing us to map the exact location of the tectonic boundary. Detrital zircon data constrain the initial closure of the Ob-Zaisan Ocean to the late Carboniferous (<323 Ma), and new structural and Ar/Ar geochronological data shed light on the subsequent collisional processes. We propose that the collisional zone between the Chinese Altai and the East/West Junggar was initially subjected to crustal thickening at ca. 323-295 Ma, followed by orogen-parallel extension (ca. 295 Ma) and transpressional deformation (ca. 286-253 Ma). The closure of the Ob-Zaisan Ocean in NW China postdates the initial phase of oroclinal bending in the western Central Asian orogenic belt, thus indicating that oroclinal bending initiated during subduction. Based on our new constraints and other available geological data, we suggest that the early stage of oroclinal bending was likely driven by slab rollback.

The Sveconorwegian Pegmatite Province – Thousands of Pegmatites Without Parental Granites

Abstract: The Late-Proterozoic Sveconorwegian pegmatite province in southern Norway and southwest Sweden hosts seven rare-element pegmatite districts with more than 5000 rare-element pegmatites. Most of these pegmatites with Niobium-Yttrium-Fluorine (NYF) signature are not related to a parental granite, but instead occur in areas of high-grade metamorphism and are the result of migmatization and local melt collection. There are three groups of pegmatites: (1) rare-element pegmatites related to H P -H T high-grade metamorphism associated with the assembly of the Sveconorwegian orogen; (2) rare-element pegmatites related to post-orogenic extension with L P -H T granulites; and (3) rare-element pegmatites related to granite magmatism during post-orogenic extension. The pegmatite formation principally comprises four periods restricted to certain tectono-metamorphic domains: (I) 1094–1060 Ma (Bamble sector); (II) 1041–1030 Ma (Idefjord terrane); (III) 992–984 Ma (Idefjord terrane, Rogaland-Hardangervidda-Telemark sector); and (IV) 922–901 Ma (Rogaland-Hardangervidda-Telemark and Bamble sectors). The observed relationships between pegmatite formation and regional high-grade metamorphism reveal that the majority of Sveconorwegian pegmatites are formed by anatexis, either by crustal stacking during different stages of continental/terrane collision (H P metamorphism) (periods I to III), or by mafic magma underplating (H T metamorphism) during orogenic extension (period IV). In several provinces that have been affected both by early H P metamorphism during continental collision and by late H T metamorphism during crustal extension, there may occur several generations of pegmatites that show mineralogical and geochemical affinity, even though they formed during several different periods. In addition, the results imply that the majority of Sveconorwegian NYF pegmatites are not necessarily formed in an anorogenic setting in relation to A-type magmatism, but in compressional or extensional orogenic settings unrelated to pluton-scale magmatism. In light of this, the genetic criteria of the pegmatite family classification scheme [NYF versus Lithium-Cesium-Tantalum (LCT)] will have to be re-evaluated.

Sedimentary record of Jurassic northward subduction of the Bangong–Nujiang Ocean: insights from detrital zircons

Abstract: The subduction polarity and related arc–magmatic evolutional history of the Bangong–Nujiang Ocean, which separated the South Qiangtang terrane to the north from the North Lhasa terrane to the south during the Mesozoic, remain debated. This study tries to reconstruct the subduction and evolution of the Bangong–Nujiang Ocean on the basis of U–Pb and Hf isotopic analyses of detrital zircons in samples from sedimentary rocks of the middle-western section of the Bangong–Nujiang suture zone in Gerze County, central Tibet. The Middle Jurassic Muggargangri Group in the Bangong–Nujiang suture zone was deposited in a deep-sea basin setting on an active continental margin. The Late Jurassic strata, such as the Sewa Formation, are widely distributed in the South Qiangtang terrane and represent deposition on a shelf. The Early Cretaceous Shamuluo Formation in the Bangong–Nujiang suture zone unconformably overlies the Muggargangri Group and was probably deposited in a residual marine basin setting. The detrital...

Three‐dimensional lithospheric S wave velocity model of the NE Tibetan Plateau and western North China Craton

Abstract: We present a new 3-D lithospheric Vs model for the NE Tibetan Plateau (NETP) and the western North China Craton (NCC). First, high-frequency receiver functions (RFs) were inverted using the neighborhood algorithm to estimate the sedimentary structure beneath each station. Then a 3D Vs model with unprecedented resolution was constructed by jointly inverting RFs and Rayleigh wave dispersions. A low-velocity sedimentary layer with thicknesses varying from 2 to 10 km is present in the Yinchuan-Hetao graben, Ordos block, and western Alxa block. Velocities from the middle-lower crust to the uppermost mantle are generally high in the Ordos block and low in the Alxa block, indicating that the Alxa block is not part of the NCC. The thickened crust in southwestern Ordos block and western Alxa block suggests that they have been modified. Two crustal low-velocity zones (LVZs) were detected beneath the Kunlun Fault (KF) zone and western Qilian Terrane (QLT). The origin of the LVZ beneath the KF zone may be the combined effect of shear heating, localized asthenosphere upwelling, and crustal radioactivity. The LVZ in the western QLT, representing an early stage of the LVZ that has developed in the KF zone, acts as a decollement to decouple the deformation between the upper and lower crust and plays a key role in seismogenesis. We propose that the crustal deformation beneath the NETP is accommodated by a combination of shear motion, thickening of the upper-middle crust, and removal of lower crust.

Building Archean cratons from Hadean mafic crust.

Abstract: Geologic processing of Earth’s surface has removed most of the evidence concerning the nature of Earth’s first crust. One region of ancient crust is the Hudson Bay terrane of northeastern Canada, which is mainly composed of Neoarchean felsic crust and forms the nucleus of the Northeastern Superior Province. New data show these ~2.7-billion-year-old rocks to be the youngest to yield variability in neodymium-142 (142Nd), the decay product of short-lived samarium-146 (146Sm). Combined 146-147Sm-142-143Nd data reveal that this large block of Archean crust formed by reworking of much older (>4.2 billion-year-old) mafic crust over a 1.5-billion-year interval of early Earth history. Thus, unlike on modern Earth, mafic crust apparently could survive for more than 1 billion years to form an important source rock for Archean crustal genesis.

Anatomy of composition and nature of plate convergence: Insights for alternative thoughts for terminal India-Eurasia collision

Abstract: The pattern and timing of collision between India and Eurasia have long been a major concern of the international community. However, no consensus has been reached hitherto. To explore and resolve the disagreements in the Himalayan study, in this paper we begin with the methodology and basic principles for the anatomy of composition and nature of convergent margins, then followed by an effort to conduct a similar anatomy for the India-Eurasia collision. One of the most common patterns of plate convergence involves a passive continental margin, an active continental margin and intra-oceanic basins together with accreted terranes in between. The ultimate configuration and location of the terminal suture zone are controlled by the basal surface of the accretionary wedge, which may show fairly complex morphology with Z-shape and fluctuant geometry. One plausible method to determine the terminal suture zone is to dissect the compositions and structures of active continental margins. It requires a focus on various tectonic elements belonging to the upper plate, such as accretionary wedges, high-pressure (HP)-ultra-high-pressure (UHP) metamorphic rocks, Barrovian-type metamorphic rocks and basement nappes, together with superimposed forearc basins. Such geological records can define the extreme limits and the intervening surface separating active margin from the passive one, thus offering a general sketch for the surface trace of the terminal suture zone often with a cryptic feature. Furthermore, the occurrence of the cryptic suture zone in depth may be constrained by geophysical data, which, in combination with outcrop studies of HP-UHP metamorphic rocks, enables us to outline the terminal suture zone. The southern part of the Himalayan orogen records complicated temporal and spatial features, which are hard to be fully explained by the classic “two-plate-one-ocean” template, therefore re-anatomy of the compositions and nature for this region is necessitated. Taking advantage of the methodology and basic principles of plate convergence anatomy and synthesizing previous studies together with our recent research, we may gain new insights into the evolution of the Himalayan orogeny. (1) The Yarlung-Zangbo ophiolite is composed of multiple tectonic units rather than a single terminal suture zone, and a group of different tectonic units were juxtaposed against each other in the backstop of the Gangdese forearc. (2) The Tethyan Himalayan Sequence (THS) contains melanges with typical block-in-matrix structures, uniform southwards paleocurrents and age spectra of detrital zircons typical of Eurasia continent. All of these facts indicate that the THS belonged to Eurasia plate before the terminal collision, emplaced in the forearc of the Gangdese arc. (3) The Greater Himalayan Crystalline Complex (GHC) and Lesser Himalayan Sequence (LHS) comprise complex components including eclogites emplaced into the GHC and the upper part of the LHS. Judging from the fact that HP-UHP metamorphic rocks are exhumed and emplaced in the upper plate, the GHC and the upper part of the LHS where eclogite occur should be assigned to the upper plate, lying above the terminal subduction zone surface. It is the very surface along which the continuous subduction of the India subcontinent occurred, therefore acting as the terminal, cryptic suture. From the suture further southward, the bulk rock associations of the LHS and Sub-Himalayan Sequence (Siwalik) show little affinity of melange, probably belonging to the foreland system of the India plate. By the anatomy of tectonic features of all the tectonic units in the Himalayan orogen as well as the ages of the subduction-accretion related deformation, we conclude that the terminal India-Eurasia collision occurred after 14 Ma, the timing of the metamorphism of the eclogites emplaced into the upper plate. The development of rifts stretching in N-S direction in Tibet and tectonic events with the transition from sinistral to dextral movements in shear zones, such as the Ailaoshan fault in East Tibet, can coordinately reflect the scale and geodynamic influence of the India-Eurasia convergence zone. By conducting a detailed anatomy of the southern Himalayas, we propose a new model for the final collision-accretion of the Himalayan orogeny. Our study indicates that the anatomy of structures, composition, and tectonic nature is the key to a better understanding of orogenic belts, which may apply to all the orogenic belts around the world. We also point out that several important issues regarding the detailed anatomy of the structures, compositions and tectonic nature of the Himalayan orogeny in future.